NMR Evidence of Freezing of Rattling Motion in LaIr2Zn20 and LaRu2Zn20

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139La nuclear magnetic resonance (NMR) measurements have been carried out to investigate structural phase transition in LaIr2Zn20 and LaRu2Zn20. The former compound undergoes second-order structural phase transition at T_{\text{s}}=200 K, whereas the latter undergoes first-order transition at T_{\text{s}}=150 K. For both compounds, narrow Lorentzian La-NMR spectra with no electric field gradient (EFG) were observed above T_{\text{s}}, indicating that the local symmetry at the La site holds a cubic symmetry. Below T_{\text{s}}, the La NMR spectrum of LaIr2Zn20 splits into more than two lines with broad tails distributed over a frequency range of 0.8 MHz, while that of LaRu2Zn20 broadens with broad tails over a frequency range of 1.6 MHz. The appearance of an EFG at the La site below T_{\text{s}} is clear evidence that the cubic local symmetry at the La site lowers below T_{\text{s}}. Both compounds show unusual increases in the nuclear spin-lattice relaxation rate at approximately T_{\text{s}}, indicating the slowing down of the fluctuation of the EFG associated with the rattling motion of Zn atoms. Below T_{\text{s}}, the lowering of the local symmetry at the La site can be ascribed to the freezing of the rattling motion.
2012-02-15